For almost all industrial applications it is necessary to prepare cellulose and cellulose ethers in the form of powder. In this context it is necessary, depending upon the specific application, to define the granule size distribution, the dry weight and, in particular, the viscosity of the cellulose- and cellulose-ether powders.
It is known that raw cellulose material which exists for example in the form of a web can be comminuted in cutting mills to a pourable coarse cellulose powder consisting substantially of individual fibers where the fiber length distribution is significantly greater than 300 μm. The coarse cellulose powder is converted in a chemical process to cellulose ethers. If necessary, the resulting cellulose ethers can similarly be ground in a ball or vibrating mill to fine powders having a granule size distribution of less than 100 μm. For other applications it is possible to grind the coarse cellulose powder to a fine cellulose powder in ball or vibration grinding mills.
During the grinding of the cellulose or cellulose ethers the mechanical energy of grinding is to a large extent converted by friction into heat energy which leads to a rise in temperature in the mill and the sieve unit. A large part of the energy is removed by air cooling. However, it is not possible to prevent the mill heating up to a temperature of 130° C. during the grinding process. As a result of the mechanical effects at an elevated temperature the cellulose-polymer molecules are partially degraded or shortened, i.e. there is a reduction in the degree of polymerization DP.
The quality of the cellulose and cellulose ethers is characterized by their viscosity levels. In essence, the viscosity is determined by the degree of polymerization DP. A number of standardized methods are available for the measurement of viscosity which produce results which differ from one another. For the cellulose used in the process according to the invention the viscosity or rather the limiting viscosity number LVN is measured by the Cuen method described in DIN 54270. This involves the cellulose being dissolved in copper-ethylenediamine at different concentrations, measurement of the relevant viscosity levels and extrapolation to zero concentration.
In principle there is great interest in grinding processes—particularly in respect of the coarse grinding of cellulose—in the course of which the degree of polymerization DP is reduced by the least possible extent.
DE 24 58 998 (=U.S. Pat. No. 4,076,935) teaches a process for the gentle fine grinding of cellulose or its ether derivatives in a vibrating mill in the presence of 5 to 14% by weight of water either already present in the starting material or added from an external source to a powder which has a granule size distribution of less than 100 μm and a residual moisture content of 2 to 10% by weight.
In DE-A 196 41 781 a process is described for the simultaneous grinding and drying of a moist material containing cellulose ether where a gas stream feeds the material into the grinding and friction space of a mill where the initial moisture content of the cellulose ether amounts to 20 to 70% and the residual moisture after the grinding and drying operation amounts to 1 to 10% by weight.
From DE-C 952 264 a three-stage process is known for converting moist, fibrous methylcellulose into powder which dissolves rapidly. In this context the product containing 50 to 70% by weight of water is first homogenized to a plastic mass, cooled to 10 to 20° C. in a refrigerated screw press and finally ground in a hammer basket mill followed by drying in a circulating air dryer.
DE-C24 10 789 (=U.S. Pat. No. 4,044,198) describes a process for the manufacture of cellulose derivatives, preferably cellulose ethers, with a high content of fine particles. Here, the moist cellulose derivatives are embrittled with liquid nitrogen and then subjected to grinding.
In EP-A 0049 815 (=U.S. Pat. No. 4,415,124) a two-stage process for the manufacture of micro-powders of cellulose ethers or cellulose is described, where the products which exhibit a fine-fibered or wool-like structure are first converted into a brittle solid form which is subjected to grinding treatment until a granule size distribution of at least 90% under 125 μm is obtained. Pellet presses or vibrating or ball mills, preferably of a refrigerated type, are used in the embrittlement stage and pinned disc or impact disc mills are used for the grinding stage.
DE-A 14 54 824 describes a process for the manufacture of granules or powders from fibrous, dry cellulose ethers by friction treatment between two rolls which run at different rotation speeds and subsequent grinding.
DE-A 30 32 778 refers to a two-stage process for the continuous homogenization of moist cellulose ethers in which the moist cellulose ethers are subjected to a cutting, impact-type and shearing action produced by orbiting rotating bodies with different cylindrical surface profiles, possibly simultaneous pelletization of the comminuted cellulose ethers with the addition of water and finally drying of the resulting pellets.
The known processes according to the state of the art are mostly of a multi-stage nature with pre-drying or pre-embrittlement or compaction and require extensive support equipment, e.g. intensive cooling. Furthermore, for all processes the thermo-mechanical stresses imposed upon the macro-molecules—and especially so when processing highly viscous, highly substituted products—are always so high that the macro-molecules are degraded by the reduction of chain length during grinding as can be seen from the more or less substantial reduction in viscosity when compared with the starting products. Furthermore, the surfaces of the products being processed are hardened by pre-embrittlement and pre-drying steps which can be seen to have a detrimental effect upon the required properties of rapid and complete solubility.
The object of the invention is to so further the development of the known processes and devices for the coarse grinding of cellulose in cutting mills that the ground cellulose powder suffers only a very small reduction in viscosity (=DP) when compared with the viscosity (=DP) of the starting product and that the temperature of the grinding device is lowered during the grinding operation.